Modern electronically controlled diesel distributor-type injection pumps control the delivery of fuel to the injectors by pumping the full stroke of the high pressure pumping plungers, and controlling the start and end of injection with a high pressure spill solenoid valve. These pumps also have "snubber" valves in the high-pressure delivery outlets. The snubber valves perform many functions, but mainly act as check valves in the delivery direction, allowing fuel to flow freely towards the injector, but controlling the return flow pressure drop in the fuel line with an orifice. This orifice allows the reflected pressure wave from the injector to dissipate back into the fuel distributor, so that the fuel delivery line pressure is dropped, preventing secondary injections.
If air is introduced into this high-pressure fuel pump section, it will collect in the high-pressure fuel line to the injector. The air will then be compressed by the action of the high-pressure pumping element and expanded back through the return flow orifice. The same compression and expansion action is repeated continuously, without the air being dispersed. As long as air is being continually compressed and expanded, insufficient injection pressure will be generated and fuel delivery through the fuel injector to the engine will be adversely affected. The result is an engine that cannot be started without mechanical (service) intervention, to bleed out the air in the high-pressure fuel line system.
If the vehicle operator allows the vehicle to run out of fuel, air will be delivered to the fuel injection system, eventually getting into the high pressure pumping chamber and injector fuel lines, causing the engine to cease operation due to fuel starvation. When the operator attempts to rectify this problem by adding fuel to the fuel tank and supply system, the air that is trapped in the high pressure pumping system will not be purged without mechanical (service) intervention.
Typically, other methods of preventing air ingress usually use the fuel tank level indicator system to warn that the vehicle is about to run out of fuel. Because of the large width to depth aspect ratio of typical vehicle fuel tanks, it is difficult to make this system accurate. Thus, there is the potential to shut down the vehicle even with measurable fuel quantity left in the tank.
The disadvantages associated with these conventional prevention methods have made it apparent that a new technique for preventing air ingestion in a diesel fuel pump is needed. Preferably, the new technique should be more accurate then a system using a fuel tank level indicator. The new system should also shut down the vehicle with as little measurable fuel left in the fuel tank as possible.